Monopole, dipole, and quadrupole borehole seismic transducers

ABSTRACT

Various configurations of cylindrically shaped transducers are used as borehole seismic wave sources. The basic class of transducers of this invention has an outer shell that is generally cylindrical, and may be comprised sections. The shell or the sections may be pre-stressed or preformed to encourage certain bending motions. Depending on the configuration of the transducer, it approximates a monopole, dipole, or quadrupole radiator. Various actuating means, including magnetostrictive forces and piezoelectric forces, are used to drive the motions of the transducer. These transducers are especially useful for generating compressional, shear, or other elastic waves in solids. For example, in geophysical applications, the transducers are placed in boreholes for either hole-to-hole or single hole measurements.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to electroacoustic transducers,and more particularly to cylindrical transducers, actuated bypiecoelectric or magnetic means, that can approximate monopole, dipole,or quadrupole acoustic radiators.

2. Description of the Related Art

Electroacoustic transducers provide a means for either generating anddetecting acoustic vibrations in various sound propagation media. Thereare many types of acoustic transducers, with a common type beingcylindrical transducers, which are characterized by a cylindrical shellthat radiates or receives acoustic vibrations.

A further distinction among cylindrical transducers is the cylindrical"bender" transducer characterized by a flexural motion, or "bending", ofthe cylinder walls when the transducer is excited. Although cylindricaltransducers, including cylindrical bender transducers, have been used togenerate and receive acoustic waves in a variety of applications, theiruse has been characteristically limited to axi-symmetrical motions ofthe cylindrical structure of the transducers.

One form of cylindrical bender transducer used to generate and detectacoustic waves in geophysical applications is described in U.S. Pat. No.4,525,646 to Shirley, et al. This transducer operates as a "selfactuating" device, with the cylindrical shell being made of at least onepiezoelectric layer. The active element of the transducer is thecomposite cylinder itself. All that is needed is the application ofpressure waves or electrical energization for the transducer to operateas a detector or source. A limitation of this device is that it isdirected toward a single motion of the cylinder walls, that motion beingaxi-symmetrical and therefore approximating a monopole source.

Geophysical exploration has long involved the use of both singleborehole and hole-to-hole seismic measurements to determinecharacteristics of the surrounding geological environment. The seismicwaves, which may be also referred to as acoustic or sonic waves, may beeither compressional or shear, as well as other types of waves.

Various means have been used for generating and detecting the seismicwaves. Most of the prior art involves the use of compressional waves,conveying by the earth's crust and detected on seismographs to giveinformation about rock structures through which they travel. Ofparticular interest are those transducers which are capable ofgenerating or detecting sound waves under conditions of deep boreholehydrostatic pressure and high temperatures. In the past, acoustictransducers have had difficulty meeting such rigorous environmentaldemands. Another requirement of such transducers, especially forhole-to-hole applications, is high power and high detection sensitivityover the frequency range of several hundred to several thousand hertz.Prior transducers have had difficulty meeting these specifications.Finally, transducers capable of preferentially generating shear waveshas been a limitation in the prior art.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an acoustic transducerthat is easily adaptable to approximate a monopole, dipole, orquadrupole acoustic radiator. Each type of radiator is particularlysuited for certain seismic wave applications. Monopole devices areprimarily used to generate and detect compressional waves. Dipoledevices are primarily used for shear waves. Quadrupole devices are usedfor higher-order wave propagation as a combination of compressional andshear waves.

Another object of the invention is to provide a source of acoustic wavesfor geophysical exploration. For such applications, a transducer isprovided that operates with high power and high frequency.

Another object of the invention is to provide a source of seismic wavesthat is useful in deep boreholes. Thus a feature of one embodiment ofthe invention is actuation of the transducers by piezoelectric means,which permits good tolerance of high temperature. A feature of a secondembodiment of the invention is actuation of the transducers withmagnetostrictive means, which permits even greater temperaturetolerance.

Another object of the invention is to provide a source of seismic wavesthat is useful for both single hole and hole-to-hole measurements. Thus,the invention provides transducers that are capable of efficientelectrical-to-acoustic energy conversion as a source and highsensitivity as a detector.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the invention, with no actuating meansin view, which is used to approximate a monopole acoustic radiator.

FIGS. 2A-2C show various methods of constraining the ends of thetransducer of FIG. 1.

FIG. 3 illustrates axi-symmetrical motion of the transducer of FIG. 1.

FIG. 4 is a cross sectional view of the transducer of FIG. 1, notincluding actuating means, wherein the cylinder walls have beenpre-formed to bend outwardly.

FIG. 5 is a cross sectional view of the transducer of FIG. 1, notincluding actuating means, showing a stress rod used to pre-stress thewalls of the transducer.

FIG. 6 illustrates the motion of the transducer of FIG. 1 when thecylinder walls are preformed or pre-stressed outwardly.

FIG. 7 is a cross sectional view of the transducer, not includingactuating means, wherein the cylinder walls have been thinned to createa preference for outward bending.

FIG. 8 illustrates the motion of a transducer whose cylinder walls havebeen thinned as in FIG. 7.

FIG. 9 is a cross sectional view of the transducer, not includingactuating means, wherein the cylinder walls have been thinned to createa preference for inward bending.

FIG. 10 illustrates the motion of a transducer whose cylinder walls havebeen thinned as in FIG. 9.

FIG. 11 is a cross sectional view of the transducer of FIG. 1, includingan actuating means comprised of a piezoelectric stack.

FIG. 12 illustrates a means for electrical connection of thepiezoelectric stack of FIG. 11, such that the transducer may beenergized.

FIG. 13 is a cross sectional view of the transducer of FIG. 1, includingan actuating means comprised of a piezoelectric stack and a means forpre-stressing the cylinder walls against the actuating means.

FIG. 14 is an alternate embodiment of the actuating means of FIG. 13.

FIG. 15 is a cross sectional view of the transducer of FIG. 1, includingan actuating means comprising a magnetostrictive driver.

FIG. 16 is a perspective view of an alternative embodiment of theinvention, with no actuating means in view, used to approximate a dipoleacoustic radiator.

FIG. 17 is a cross sectional view, not including an actuating means, ofthe transducer of FIG. 16 along section lines 17--17.

FIG. 18 illustrates the motion of the transducer of FIG. 17.

FIG. 19 is a perspective view of an alternative embodiment of theinvention, with no actuating means in view, used to approximate aquadrupole acoustic radiator.

FIG. 20 is a cross sectional view at section 20--20, not including anactuating means, of the transducer of FIG. 19.

FIG. 21 illustrates a cross-sectional view at Section 21--21, notincluding an actuating means, transducer of FIG. 19.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The transducer invention described herein has a number of differentembodiments. Although all embodiments share structural similarities,such as at least one cylindrical bending member, which is or forms agenerally cylindrical shaped shell, the embodiments may be technicallycategorized by the type of wave that is produced. Thus, three categoriesof transducers are described below: monopole, dipole, and quadrupole.These three categories are described below in Parts A, B, and C,respectively. All three types are useful for measurements in a singleborehole, such as for sonic velocity measurements. The monopole anddipole transducers are also useful for hole-to-hole seismic imagingmeasurements, which are capable of characterizing large volumes of therock formations of interest.

These three categories of transducers can be further varied according toother structural variations, such as pre-stressing means or actuatingmeans. Generally, these variations can be used with either a monopole,dipole, or quadrupole transducer. For convenience, they are describedbelow in connection with monopole transducers, with subsequentreferences to those descriptions for the purpose of describing dipoleand quadrupole transducers.

A. MONOPOLE TRANSDUCERS

FIG. 1 shows an embodiment of the invention configured as a monopoletransducer 10. Although this sonic monopole transducer 10 could be usedfor a number of applications, it is especially well suited for use in aborehole in the earth for the purpose of measuring compressional wavevelocity and other parameters of the rock formation surrounding theborehole.

The monopole transducer 10 is the simplest embodiment of one of thefeatures of the invention, which is the driving of a cylindrical shellor cylindrical sections that approximate a cylindrical shell with anactuating means that provides an axial force on the cylinder so as tocause radially-oriented flexural deflection of the wall of the shell orwalls of the sections. For monopole acoustic radiation, the radialdeflection is azimuthally uniform about the axis of the cylinder andhence is axially symmetrical.

Transducer 10 has three basic parts: a hollow shell 12, end caps 14a and14b, and an actuating means. The actuating means is shown in FIG. 1, butis shown and discussed below in connection with FIGS. 11-15.

Shell 12 is cylindrical in shape. This cylindrical shape is preferredfor use in boreholes, but polygonal approximations of a cylindricalshape may also be used. Shell 12 is made of metallic, ceramic, or othermaterial having mechano-elastic characteristics comparable to those ofthe actuator component of the device.

The thickness of the wall of shell 12 is small compared to the radius.The length of shell 12 is large compared to its width. The length,width, and thickness of shell 12 affect the frequency range andsensitivity of the transducer.

Each end of shell 12 is constrained by end caps 14a and 14b. End caps14a and 14b are more massive and rigid than shell 12, and therefore areinertial masses that constrain the motion of the shell 12 at its ends.FIGS. 2A-2C depict various means for constraining the ends of shell 12with end caps 14a and 14b. FIG. 2A shows the ends rigidly fixed. FIGS.2B and 2C show the ends pinned. Which of these three methods is used maydepend on whether shell 12 is pre-stressed by any of the various meansdiscussed below. If shell 12 is pre-stressed in compression, any of theconstraining means shown in FIGS. 2A, 2B, and 2C may be used. However,if shell 12 is pre-stressed in tension, the fixed end means of FIG. 2Ais used. The embodiments described herein use the fixed end method ofFIG. 2A, although, except for the pre-stressed in tension embodiments,the other methods could be used.

FIG. 3 is a cross sectional view of shell 12 and end caps 14a and 14b,showing that the structure of transducer 10 is axi-symmetrical. Althoughin FIG. 3 the thickness of the cylindrical wall of shell 12 is ofuniform thickness, this is not a necessary requirement and, as discussedbelow, the motion of transducer 10 may be preferentially controlled ifthe thickness is varied at the midsection of shell 12.

FIG. 3 also shows the motion of transducer 10 when the actuating means,which is described below in connection with FIGS. 11-15, is energized.The forces resulting from the actuating means are indicated by the forcearrows, A and B. As discussed below, different embodiments of thetransducer use different actuating devices, but in all embodiments, theforce resulting from the actuating means is axial, as indicated by forcearrows A and B. Furthermore, although force arrows A and B are shown asacting at only one end of the transducer, the actuating means may alsoprovide the same forces as both ends.

For all embodiments of the monopole transducer 10, the motion of shell12 is axisymmetrical. Thus, if a compressing axial force A is applied totransducer 10 with its base fixed, shell 12 will bulge or expand inoutward axi-symmetrical flexure at its midsection with its endsremaining fixed. If the force is reversed, and a tensile axial force Bis applied, elongation of transducer 10 will be accompanied by an inwardflexing of shell 12, again with the ends remaining fixed. If the axialforce A and B are oscillatory, the flexural deflections of shell 12 willalso be oscillatory.

FIGS. 4-10 show several means for varying the motion of transducer 10from the motion shown in FIG. 3. These means include: pre-forming shell12 by changing its curvature as in FIG. 4, pre-stressing shell 12 as inFIG. 4, or thinning the walls of shell 12 as in FIGS. 7 and 8. Asexplained below, these modifications affect the manner in which theapplied axial force will cause the cylinder walls of shell 12 to move.Furthermore, pre-forming, pre-stressing, and thinning the shell wallsare not exclusive of each other; more than one of these modificationscan be used on a single transducer to result in a desired motion.

In FIG. 4, shell 12 has been fabricated to have an outward curvature,which causes a preference for outward motion. It is also possible toshape shell 12 so that it has an inward curvature, which would cause apreference for inward motion.

In FIG. 5, shell 12 has been pre-stressed to have an outward curvature.This pre-stress may be the result of a static compressional force thatcauses a static outward deflection of shell 12. One method ofintroducing this pre-stress is to include a stress rod 52 inside shell12, along the axis of shell 12, and a means for tensioning stress rod52. The ends of stress rod 52 are attached to a rigid body that issufficient in size and shape to provide a bearing surface at each end ofshell 12. As shown in FIG. 5, this attachment may be to end caps 14a and14b. The means for tensioning stress rod 52 may be as simple as atightening nut at each end of stress rod 52. Thus each end cap 14a and14b has a hole in its center, through which stress rod 52 extends.Stress rod 52 is threaded at each end, and the ends of stress rod 52 aresufficiently long to extend from both end caps 14a and 14b. Tighteningnuts 44a and 44b are threaded onto each end of stress rod 42 and may beturned to apply tensile force to stress rod 52. This translates to astatic compressional force on shell 12, which causes shell 12 to beoutwardly deflecting.

FIG. 6 illustrates the motion of transducer 10 when shell 12 ispreformed or pre-stressed outwardly. When transducer 10 is operated, itdeflects outward and inward, but tends to exhibit a preference foroutward motion.

FIGS. 7 and 9 show another means for varying the deflection oftransducer 10 from the motion shown in FIG. 3. As shown in FIGS. 7 and9, the wall thickness of shell 12 has been made thinner at themidsection than at the ends. In FIG. 7, the wall thickness has been madethinner at the midsection by increasing the inside diameter of shell 12.This modification allows the midsection of shell 12 to deflect outwardfarther under an applied axial compressional force than in the case of auniform cylinder wall, as shown in FIG. 8. In FIG. 9, the wall thicknesshas been made thinner at the midsection by decreasing the outsidediameter of shell 12. This modification creates a bending moment betweenthe midsection of shell 12 and its ends, and thereby allows themidsection of shell 12 to deflect inward under an applied axialcompression, as shown in FIG. 10.

FIGS. 11-15 show various actuating means for the flexing motion oftransducer 10. Each of these methods provides a compressional or tensileforce, which is axial to shell 12, and acts on the walls of shell 12 atone or both ends of shell 12. In the preferred embodiment, the axialforce is uniform around the circumference of shell 12. This may beaccomplished by applying the force against end caps 14a and 14b,although any other structure that bears upon the shell walls in asimilar manner may be used.

The axial forces on the walls of shell 12 cause the cylinder walls ofshell 12 to flex outward or inward as discussed above. Generally, theactuating means are either piezoelectric or magnetostrictive devices.The piezoelectric devices are able to tolerate high temperatures inboreholes and have high electromechanical transduction efficiency. Themagnetostrictive devices have the same desirable operatingcharacteristics in even higher tolerance of extreme borehole temperatureconditions.

The actuating device shown in FIG. 11 comprises a piezoelectric stack112. The piezoelectric material of the preferred embodiment is ceramic,which is operates well under high temperature conditions. Stack 112 ismounted inside shell 12 along the axis of shell 12. Stack 20 iscomprised of a number of piezoelectric disks 114a-114n, one atop theother. Disks 114a-114b are polarized so that their thickness varies whenelectrically excited. Stack 112 is pre-stressed in compression insideshell 12, and is thus always in firm contact with end caps 14a and 14b,regardless of whether stack 112 is energized to contract or expand.Thus, a static tensile force exists within shell 12 on end caps 14a and14b, together with a static compressional force on stack 112.

When electrically excited to expand, stack 112 causes an outward forceon end caps 14a and 14b, which causes shell 12 to deflect inward.Contraction of stack 112 causes the tensile force in shell 12 todecrease, which causes shell 12 to deflect outward. Electricalexcitement of stack 20 can cause it to expand and contract sequentially,thereby imparting an oscillatory driving force to produce deflections oftransducer 10.

FIG. 12 illustrates the manner in which disks 114a-114n are assembled inmechanical series and connected in electrical parallel. This permitslarge axial piezoelectric response of stack 112 to be produced withrelatively low excitation voltages.

FIGS. 13 and 14 show two alternative means for applying axial force toshell 12. Although both of these alternatives make use of at least onepiezoelectric stack, shown as 132 in FIG. 13 and 142a and 142b in FIG.14. A significant feature is that shell 12 and the stack 132 or stacks142a and 142b are pre-stressed in static compression by means of atensile stress rod 52. The purpose of this pre-stress is to maintaincontact between piezoelectric stack 132a or stacks 142a and 142b and atleast one end of shell 12. As explained above in connection with FIG. 5,stress rod 52 is placed along the axis of shell 12, and the tensionimparted by stress rod 52 results from the tightening of end nuts 54aand 54b.

The disks from which the piezoelectric stacks 132, 142a, and 142b aremade are annular in shape to accommodate stress rod 52. The disks areconfigured to expand or contract in thickness when electricallyenergized, which causes the stack to lengthen or shorten. This in turn,causes an axial force on the walls of shell 12 at that end of shell 12.In FIG. 13, piezoelectric stack 132 is adjacent to one end of shell 12,thus force is applied at one end of shell 12. In FIG. 14, piezoelectricstacks 142a and 142b are at both ends of shell 12, thus force is appliedat both ends of shell 12. The disks of stacks 44, 46a, and 46b may beelectrically connected in a manner similar to that shown in FIG. 12.

If the walls of shell 12 are uniform in thickness or are thinned byincreasing the inside diameter of shell 12, the compressionalpre-stressing resulting from stress rod 42 causes a preference foroutward deflection. Alternatively, the walls of shell 12 may be thinnedby decreasing the outer diameter of shell 12, thereby permitting inwarddeflection. FIG. 15 shows another type of actuating device that ismagnetostrictive rather than piezoelectric. The magnetostrictiveelements are shell 152, permanent magnets 154a and 154b, and coil 156.Like the piezoelectric embodiments of FIGS. 13 and 14, a tension rod 158along the axis of the transducer imparts a compressional pre-stress onshell 152. Magnets 154a and 154b bias the circuit and provide a staticflux density, whereas coil 156 is used to induce a magnetic flux.

Magnet 154a is fixed at one end of shell 152 and magnet 154b is fixed atthe other end. Preferably, magnets 154a and 154b are annular disks, topermit tension rod 152 to pass through them. For proper biasing, magnets154a and 154b are each radially magnetized, with the magnetization ofmagnet 154a being opposite to that of magnet 154b. For example, ifmagnet 154a is polarized with its outer diameter as N and its innerdiameter as S, magnet 154b should be oppositely polarized.

Shell 152 and tension rod 158 are concentrically arranged, with shell152 being made of a permeable metal. More specifically, shell 152 ismade of a magnetostrictive material having a positive magnetostrictivecoefficient, such as the alloy Alfer, which is 13% aluminum and 87%iron. Tension rod 158 is made from a magnetostrictive material having anegative magnetostrictive coefficient, such as nickel. Alternatively,the material for shell 152 and tension rod 158 can be negative for shell152 and positive for tension rod 158. To avoid eddy current losses andoptimize operating efficiency, tension rod 158 should be constructedusing length oriented laminations.

Coil 156 is electrically conductive and is wrapped around a portion ofthe outer diameter of tension rod 158. By introducing an electricalcurrent in coil 156, a magnetic flux is induced in the magnetic circuitand shell 152 is made to deflect.

In FIG. 15, the transducer is outward deflecting as a result of thethinner wall thickness and the increased inner diameter of shell 152.Alternatively, the transducer could be made inward deflecting if theouter diameter of shell 152 were decreased as in FIG. 9.

B. DIPOLE TRANSDUCER

FIGS. 16-19 show an embodiment of the invention that approximates asonic dipole source. This embodiment, shown as transducer 160, combinesthe features of the outward and inward deflecting embodiments of FIGS. 7and 9. An important feature of the dipole transducer is the generationof asymmetrical motion. In geophysical applications, when the dipoletransducer is placed in a borehole and energized, it generateshorizontally polarized shear waves in the surrounding rock formations.The measurements from these waves can be used to determine the shearwave velocity and other parameters of the geological formation beingtested.

The dipole transducer replaces the cylindrical shell of the monopoleembodiments with two cylindrical sections 162 and 164, which are eachless than a half cylinder and are preferably slightly less than a halfcylinder. The ends of cylindrical sections 162 and 164 are constrainedby end caps 166a and 166b, in a manner similar to that discussed abovein connection with FIGS. 2A-2C. Cylindrical sections 162 and 164 arearranged in a fixed position so that their combined shape approximates acylinder.

As shown in FIG. 17, flexing members 162 and 164 are constructed so thatthey are oppositely deflecting. This is accomplished in a manner similarto the construction of the embodiments shown in FIGS. 7 and 9, but usinghalf cylindrical sections rather than cylinders. Thus, the innerdiameter of one cylindrical section 162 or 164 is increased, and theother diameter of the other cylindrical section 162 or 164 is decreased.

FIG. 18 is a cross sectional view of the dipole transducer in motion,when energized by an actuating means (not shown). The actuating meansmay be any of the actuating means discussed above in connection withFIGS. 8-15. An oscillating force resulting from the actuating means,such as indicated by force arrows A and B, will result in diametricallyopposing asymmetrical deflections of the dipole transducer. Thesedeflections are asymmetrical with respect to the axis of transducer 160,and permits transducer 160 to approximate the operation of an acousticdipole source. For example, dipole transducer 160 may include theactuating and compressional pre-stressing means of FIG. 13. In thisconfiguration, the axial expansion of piezoelectric stack 44 will imposean additional compressional stress on the two flexing members 162 and164 causing them to be further deflected outwardly and inwardly as shownin FIG. 18.

C. QUADRUPOLE TRANSDUCERS

FIGs. 19-21 show an embodiment of the invention that approximates asonic quadrupole transducer. This embodiment, shown as transducer 190,combines the features of the outward and inward deflecting embodimentsof FIGS. 7 and 9 and the features of the dipole transducer 160. Afeature of the quadrupole transducer is the generation of a combinationof compressional waves and shear waves in surrounding rock formationswhen the transducer is placed in a borehole. A particular mode ofelastic wave propagation in the drilled rock formation, as excited by aquadrupole source transducer, is known as the "screw" or "helical" mode,which is especially useful in determining the presence of fractures andanisotropic geological conditions surrounding the borehole.

In the quadrupole transducer embodiment 190, the cylindrical shell ofthe monopole embodiments is replaced with two pairs of cylindricalsections 192a and 192b and 194a and 194b, which are each less than aquarter cylinder and are preferably slightly less than a quartercylinder. Cylindrical sections 192a, 192b, 194a and 194b are arranged infixed positions so that their combined shape approximates a cylinder.Cylindrical section 192a is diametrically opposed to the other member ofits pair, cylindrical section 192b. Similarly, cylindrical section 194ais diametrically opposed to the other member of its pair, cylindricalsection 194b.

Flexing members 192a, 192b, 194a, and 194b are shaped or pre-stressed orboth to be oppositely deflecting in two orthogonal planes, C and D.FIGS. 20 and 21 are cross-sectional views of the quadrupole transducerin these two planes. This construction is similar to the construction ofthe embodiments shown in FIGS. 7 and 9, but using half cylindricalsections rather than cylinders. Thus, the inner diameter of one pair ofcylindrical sections 192a and 192b is increased, and the outer diameterof the other pair of cylindrical sections 194a and 194b is decreased.

The ends of each flexing member 192a, 192b, 194a, and 194b are attachedto end caps 196a and 196b, in a manner similar to that discussed abovein connection with FIGS. 2A-2C.

The actuating means of the quadrupole transducer is now shown in FIGS.19-21, but may be any of the actuating means discussed above inconnection with FIGS. 8-15. An axially oriented oscillating forceresulting from the actuating means applied to the transducer 190 willcause outward deflections in one plane and inward deflections in asecond plane oriented at 90 degrees with respect to the first plane. Forexample, if the transducer of FIGS. 14-16 includes the actuating andcompressional pre-stressing means of FIG. 13, the axial expansion ofpiezoelectric stack 45 will impose an additional compressional stress onthe four flexing members 192a, 192b, 194a, and 194b, causing them to befurther deflected outwardly and inwardly.

D. RECEIVER TRANSDUCERS

The descriptions above are directed toward the use of the invention as asource transducer, which is actuated to produce bending of the cylinderwalls. A feature of the invention, however, is that each of thetransducers operate reciprocally. In other words, pressure waves againstthe cylinder walls cause them to bend, which causes a voltage to begenerated, by the reciprocal action of the piezoelectric ormagnetostrictive transducer mechanisms. When used as receivingtransducers, they can be configured and oriented so that they aresensitive to a desired type of wave, such as compressional or shear orother wave type.

Although the invention has been described with reference to specificembodiments, this description is not meant to be construed in a limitingsense. Various modifications of the disclosed embodiment, as well asalternative embodiments of the invention, will become apparent topersons skilled in the art upon reference to the description of theinvention. It is therefore contemplated that the appended claims willcover such modifications that fall within the true scope of theinvention.

I claim:
 1. A cylindrical transducer for approximating a dipole acousticradiator, comprising:a first cylindrical section, having a wall that isrelatively thin compared to its radius, said wall being thinned at themidsection by decreasing the outer diameter of said cylindrical section,said thinned wall imparting a preference of said first cylindricalsection to flex inwardly when longitudinally compressed, saidcylindrical section having a top and a bottom end, a second cylindricalsection, having a wall that is relatively thin compared to its radius,said wall being thinned at the midsection by increasing the innerdiameter of said cylindrical section, said thinned wall imparting apreference of said second cylindrical section to flex outwardly whenlongitudinally compressed, said cylindrical section having a top and abottom end, means for attaching said first and second cylindricalsections in a fixed position such that together their shape approximatesa cylindrical shell, an actuating means for providing an axial force atleast one of said ends of said cylindrical sections, inertial masses ateach of said ends of said cylindrical sections for constraining themovement of said shell at said ends, and means for connecting saidactuating means to said cylindrical sections so that energization ofsaid actuating means causes said cylindrical section walls to bendasymmetrically relative to the axis of said cylindrical shell.
 2. Thetransducer of claim 1 wherein said cylindrical section walls arepreformed to curve inwardly or outwardly, thereby imparting a preferenceof said walls to flex inwardly or outwardly.
 3. The transducer of claim1 further comprising means for pre-stressing said cylinder walls incompression.
 4. The transducer of claim 3 wherein said pre-stressingmeans comprises a tension rod along the axis of said cylindrical shell.5. The transducer of claim 1 wherein said actuating means comprises astack of piezoelectric disks axially oriented inside said cylindricalshell, configured such that said stack expands or contractslongitudinally when electrically energized.
 6. The transducer of claim 1wherein said actuating means comprises a stack of piezoelectric disksaxially oriented outside said cylindrical shell, adjacent to one of saidends of said cylindrical shell, configured such that said stack expandsor contracts longitudinally when electrically energized, and a means forpre-stressing said actuating means against said cylindrical shell incompression, such that contact between said shell and said actuatingmeans is maintained during said expansion and contraction.
 7. Thetransducer of claim 1 wherein said actuating means comprises amagnetostrictive circuit.
 8. The transducer of claim 7 wherein saidmagnetostrictive circuit comprises a permanent magnet at each end ofsaid cylindrical shell and a magnetic flux providing coil.
 9. Thetransducer of claim 1 wherein said inertial masses, said attachingmeans, and said connecting means are end caps at the ends of saidcylindrical shell.
 10. The transducer of claim 1 wherein said transducerreceives rather than generates acoustic waves, and thus said actuatingmeans responds to, rather than creates, bending motion of said cylinderwalls and said means for connecting said actuating means to saidcylindrical shell permits said actuating means to respond to saidbending.
 11. A cylindrical transducer for approximating a quadrupoleacoustic wave source, comprising:a first and a second cylindricalsection, each having a wall that is relatively thin compared to itsradius, said walls being thinned at the midsection by decreasing theouter diameter of said section, said cylindrical sections having a topand a bottom end, a third and a fourth cylindrical section, each havinga wall that is relatively thin compared to its radius, said walls beingthinned at the midsection by increasing the inner diameter of saidcylindrical section, said cylindrical sections having a top and a bottomend, means for attaching said first and third and said second and fourthcylindrical sections diametrically opposed to each other, in a fixedposition, such that together said cylindrical sections approximate acylindrical shell, an actuating means for providing an axial force atone of said ends of said cylindrical sections, inertial masses at eachof said ends of said cylindrical sections for constraining the movementof said shell at said ends, and means for connecting said actuatingmeans to said cylindrical sections so that energization of saidactuating means causes said cylindrical section walls to bendasymmetrically with respect to the axis of said cylindrical shell. 12.The transducer of claim 11 wherein said cylindrical section walls arepre-formed to curve inwardly or outwardly, thereby imparting apreference of said walls to flex inwardly or outwardly.
 13. Thetransducer of claim 11 further comprising means for pre-stressing saidcylinder walls in compression.
 14. The transducer of claim 11 whereinsaid pre-stressing means comprises a tension rod along the axis of saidcylindrical shell.
 15. The transducer of claim 14 wherein saidpre-stressing means comprises a tension rod along the axis of saidcylindrical shell.
 16. The transducer of claim 12 wherein said actuatingmeans comprises a stack of piezoelectric disks axially oriented insidesaid cylindrical shell, configured such that said stack expands orcontracts longitudinally when electrically energized.
 17. The transducerof claim 12 wherein said actuating means comprises a stack ofpiezoelectric disks axially oriented outside said cylindrical shell,adjacent to one of said ends of said cylindrical shell, configured suchthat said stack expands or contracts longitudinally when electricallyenergized, and a means for pre-stressing said actuating means againstsaid cylindrical shell in compression, such that contact between saidshell and said actuating means is maintained during said expansion andcontraction.
 18. The transducer of claim 11 wherein said actuating meanscomprises a magnetostrictive circuit.
 19. The transducer of claim 18wherein said magnetostrictive circuit comprises a permanent magnet ateach end of said cylindrical shell and a magnetic flux providing coil.20. The transducer of claim 11 wherein said inertial masses, saidconnecting means, and said attachment means are end caps on said ends ofsaid cylindrical shell.